A configuration of a general fuel cell unit cell (also referred to as a unit cell) and, particularly, a configuration of a main part of the fuel cell unit cell including electrode sections, will be outlined below. As shown in FIG. 3, the so-called membrane electrode assembly (MEA) is configured by providing a cathode catalyst layer 12 (also referred to as an oxidizing electrode or a cathode electrode) and an anode catalyst layer 14 (also referred to as a fuel electrode or an anode electrode) so as to face each other with an electrolyte membrane 10 thereinbetween, and further by providing a cathode diffusion layer 16 and an anode diffusion layer 18 outside the cathode catalyst layer 12 and the anode catalyst layer 14, respectively. Further, a unit cell 50 is formed by, for example, adhesively bonding a cathode side separator 26 which is provided outside the cathode diffusion layer 16 and in which an oxidizing gas flow path 20 and a cell refrigerant flow path 22 are formed, and an anode side separator 28 which is provided outside the anode diffusion layer and in which a fuel gas flow path 24 and a cell refrigerant flow path 22 are formed, in an integrated manner.
In the unit cell 50 shown in FIG. 3, electricity is generated by supplying, as reactant gases, an oxidizing gas containing at least oxygen, such as oxygen or air, to the cathode catalyst layer 12, and a fuel gas containing at least hydrogen, such as hydrogen or a reformed gas, to the anode catalyst layer 14. Because, in such a fuel cell, heat is normally generated in a chemical reaction during electricity generation, the fuel cell is prevented from being overheated by causing a refrigerant such as water or ethylene glycol to flow through the cell refrigerant flow paths 22 shown in FIG. 3, thereby controlling the fuel cell to be within a predetermined temperature range, such as approximately 60 degrees to 100 degrees.
FIG. 4 shows an example schematic configuration of a general cell stack formed by stacking a plurality of unit cells 50 shown in FIG. 3. Because FIG. 4 is used for an explanation of the flow of a fluid (containing a reactant gas (an oxidizing gas or a fuel gas) and a refrigerant) which is supplied from the outside to the inside of the cell stack and discharged therefrom, the detailed configuration of the unit cell 50 including, for example, the separators and the MEA is omitted or described only briefly.
In FIG. 4, a cell stack 300 is usually a stack of a plurality of unit cells 50 which are necessary to obtain a desired electricity generation performance. The cell stack 300 (also referred to as a fuel cell stack) is configured such that the entire stack of the unit cells 50 is pressed and held from the outside of the both ends of the unit cell 50 stack along the stacking direction, and is fixed in place by, for example, fastening bolts (not illustrated).
Further, in FIG. 4, a fluid supply manifold 132 and a fluid discharge manifold 136 are formed so as to penetrate through each of the unit cells 50. After a fluid is supplied from outside, for example, as indicated by an arrow 134, the fluid flows through inside a fluid flow path (not illustrated herein), thereby being used for cell reaction or heat exchange in electrode sections (not illustrated) of the unit cell stacks 50 and then discharged to the outside, as indicated by an arrow 138.
In FIG. 4, the fluid supply manifold 132 is at least one of three independent supply manifolds for different types of fluids; that is, one of a fuel gas supply manifold, an oxidizing gas supply manifold, and a refrigerant supply manifold. Similarly, the fluid discharge manifold 136 is one of three independent discharge manifolds which correspond to the fluid supply manifold 132; that is, one of a fuel gas discharge manifold, an oxidizing gas discharge manifold, and a refrigerant discharge manifold.
Specifically, in the cell stack or the fuel cell stack 300 shown in FIG. 4, the fuel gas supplied from the fuel gas supply manifold (132) is distributed to fuel gas flow paths (not illustrated) formed in the unit cells 50 (corresponding to the fuel gas flow path 24 formed in the unit cell 50 shown in FIG. 3) and used for cell reaction in the unit cells 50, and then discharged, as off-gas, from the fuel gas discharge manifold (136). Meanwhile, the oxidizing gas supplied from the oxidizing gas supply manifold (132) is distributed to oxidizing gas flow paths (not illustrated) formed in the unit cells 50 (corresponding to the oxidizing gas flow path 20 formed in the unit cell 50 shown in FIG. 3) and used for cell reaction in the unit cells 50, and then discharged, as off-gas, from the oxidizing gas discharge manifold (136). Further, the refrigerant supplied from the refrigerant supply manifold (132) is distributed to refrigerant flow paths (not illustrated) formed in the unit cells 50 (corresponding to the cell refrigerant flow paths 22 formed in the unit cell 50 shown in FIG. 3) and used for heat exchange with the unit cells 50, and then discharged from the refrigerant discharge manifold (136).
In order that the electrolyte membrane 10 performs a predetermined function as a fuel cell in the unit cell 30 shown in FIG. 3, the electrolyte membrane 10 must function as a proton conductive electrolyte membrane, and, for this purpose, it needs to maintain at least an amount of moisture greater than a predetermined amount of moisture. It is therefore a common practice to maintain a certain amount of moisture in the electrolyte membrane 10 by, for example, supplying into the unit cell 50 a fuel gas and/or an oxidizing gas that are humidified in advance to contain a predetermined amount of moisture (these are sometimes generically referred to as reactant gases).
Meanwhile, the temperature of the fuel cell stack normally drops to around room temperature during downtime. Therefore, when the humidified reactant gases are caused to flow during the operating period as described above, the moisture in the reactant gases remaining in each of the reactant gas supply and discharge manifolds may be condensed. If the amount of condensed moisture is large and water cannot be discharged, the so-called flooding in which the manifolds and the fluid flow paths are blocked with the moisture may occur. Further, particularly during the cold period, such condensed water may be frozen in the flow paths, and restarting operation may take time.
In response to this, as shown in FIG. 5, a combined pair of cell stacks of a first cell stack 400a and a second cell stack 400b; that is, a configuration in which the number of laminations of the unit cells are divided into two parts, can be used. According to the present embodiment, it is possible to acquire the necessary electromotive force in the entire pair of cell stacks, while reducing the amount of accumulated condensed water by shortening the discharge distance for the condensed water; more specifically, the length of the reactant gas supply/discharge manifolds.
However, simply reducing the number of laminates of the unit cells may still be insufficient, because discharge of the condensed water containing produced water depends on a flow volume, a flow rate, and a temperature of off-gas flowing through each of the reactant gas manifolds.
Patent Document 1 discloses stacks that are positioned in a V-shape when viewed from the unit cell stacking direction, in order to discharge hydrogen remaining in a stack case.
Patent Document 2 discloses a fuel cell stack which is inclined at a predetermined angle in order to create balance in design, and which has a short tube for effectively extracting the air entrained in a cooling water flow path.
Patent Literature 1: JP 2005-158339 A
Patent Literature 2: JP 2007-103082 A